1. Field of the Invention
The invention relates to a control apparatus and method for controlling thrust and attitude of an aircraft provided with two gas-turbine engines.
2. Description of the Related Art
Published Japanese Translation of PCT application JP-T-08-502805 describes a technology related to a helicopter provided with two free turbine engines. According to the technology, when a failure is detected in one of the two engines, the upper limit of operable range for the remaining engine is increased.
Aircraft, such as a helicopter, that obtains thrust using two engines are configured to ensure safe operation even in the event of an engine failure. Even if a failure occurs in one of the two engines, the aircraft can continue flying using the remaining engine. Accordingly, the engine output is set, basically, to a value at which the aircraft can continue flying safely using only one engine.
When both engines are operating properly, the output required by the aircraft is produced by the two engines. Accordingly, in this case, the output from each one of the two engines is lower than the output when only one engine is used.
The fuel economy of gas turbine engines operating at a low load is very poor. Accordingly, it is an important object to improve the fuel economy when both engines of the aircraft operate at low load.
In order to improve the fuel economy of the engines, an OEI (One Engine Inoperative) output (i.e., an increased engine output that is used only in the event of an emergency) from the engine may be used when an emergency arises, for example, when a failure occurs in one of the two engines. In this method, when a failure occurs in one of the two engines, the engine speed and the turbine inlet temperature are increased above the levels when the engine produces the rated output, in the remaining engine that operates properly. A greater engine output is then produced, which enables the aircraft to continue flying using only one engine.
As described above, in an engine configured on the assumption that the OEI output (emergency output) may be used, if an emergency arises, the engine output is increased by increasing the engine speed and the turbine inlet temperature above the levels when the engine runs to produce the rated output. Accordingly, unless the engine, speed and the turbine inlet temperature increase to predetermined values, the required output cannot be produced. Therefore, in the event of an emergency, it is necessary to enable the engine to produce the OEI output as soon as possible.
However, in a system configured on the assumption that the OEI output is used in the event of an emergency, immediately after a failure occurs in one of the two engines, the engine speed of the remaining engine that operates properly has not increased to the predetermined value yet. Accordingly, if the output required by the aircraft is greater than the output produced by the engine, the engine is overloaded, which prevents the engine speed from increasing to the value at which the OEI output can be produced. Alternatively, there may be a considerable delay before the OEI output is produced.